Switching mode power supply (SMPS) is widely used to convert an input voltage into an output voltage by controlling the switching of at least one switch. In one control method, a switch of a buck converter is turned on once a predetermined condition is detected, for example, when a feedback signal indicative of the output voltage drops below a reference signal. The switch keeps in ON state with an on time period, and at the end of the time period, the switch is turned off.
Usually, the on time period of the switch during light load is the same with that during heavy load. And during light load, when the inductor current reaches zero, a synchronous rectifier is turned off to prevent the discharge of the output voltage. Accordingly, all the energy stored in the inductor is dumped into the output capacitor, and the output ripple of the output voltage is much larger than that during heavy load. And this is undesirable.
One traditional way to reduce the output ripple in light load is to adopt a large output capacitor, but this increases the size and also increases the system cost.
Accordingly, an improved method is desired to at least overcome one of the above mentioned deficiencies.